There
is Linux support for PPPoE. At the moment, a good choice is the
Roaring Penguin PPPoE software (available freely). For the future, PPPoE support will be in the 2.4.x series of the Kernel, currently slated for December. Any linux distro will do really (they can all be configured to help with internet sharing), my personal favorite is
Slackware, but it's often said that it's not for the "faint of heart" whatever that means. More than likely the document you'd be interested in is the HOWTO document on "IP Masquerading"... which can be had at the
Linux Documentation Project. Good luck!
EDIT: Note that while all distrobutions are available for download freely, some lag behind in updating their FTP servers, and with several companies, the downloaded version lacks key packages, which means time wasted locating and compiling, unless you can find RPMs, which still means time lost locating